I have started playing with S5, specifically Pod::S5 (search.cpan.org/dist/Pod-S5/S5.pm). It looks fairly nice, but I none of the default themes are to my liking and I don't really want to spend the time hacking up a CSS file to make it look nice. Right now it is my fall back position.
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Chas. OwensJun 17 '10 at 14:53

Good idea, but I am not guaranteed a network connection. I have changed the question to reflect this requirement.
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Chas. OwensJun 17 '10 at 19:23

Note that you can download finished presentations for offline use: prezi.com/learn/portable-prezi There's even an offline editor (Prezi Desktop), but it needs a paid subscription.
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sleskeJun 17 '10 at 21:53

@sleske Ah, good news, I saw that it was a web app and stopped looking. I will have to give it another look.
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Chas. OwensJun 17 '10 at 22:14

NeoOffice is free and opensource, works great on the Mac, and can export to PowerPoint, PDF, and various other formats (including, importantly, HTML for website viewing) for online and offline consumption. I would recommend this.

I've hear good things about 280 Slides, which has a web interface that looks and uses like Apple Keynote, but of course runs on all platforms, and runs quickly. I seriously believe it supports downloading slides for offline use in Microsoft PowerPoint format, etc.